As one of outputting devices for personal computers or facsimiles, there is the printer. Together with the remarkable popularization of computer systems, printers for recording their outputs by visualization have been developed markedly. With the progress and development of technology, various recording systems have been practically applied, including the dot system, the typewriter system, the electrostatic copying system, the plotter system, etc., but the leading system is the dot system.
Of the dot systems, there are the ink jet system and the heat transfer recording system using a heat-sensitive transfer ribbon. Particularly, heat-sensitive transfer recording has been expected to be most promising for its maintenance-free characteristic, capability of using plain paper, speed-up suitability, easy reduction in cost of the device, etc.
However, in heat-sensitive transfer recording, there are still many demands for improvement of performance and quality by the user concerning the heat-sensitive transfer ribbon to be used. In general, heat-sensitive transfer recording uses a ribbon coated with a heat-fusible ink on one surface of a base film, namely, the system in which printing is effected by application of high heat on the thermal head. In this method, it is required that application of high heat by the thermal head be immediately transmitted to the heat-fusible ink layer on the opposite surface. For this purpose, the base film holding the heat-fusible ink layer is required to be made as thin as possible. However, when a synthetic resin film such as polyester film is made into a thin film, there is the problem that its strength and heat resistance will be inevitably lowered. Also, for corresponding to speed-up of recording, there is adopted the method in which the application time of heat on the thermal head is shortened by increasing the voltage applied, but in this case the surface temperature of the thermal head may sometimes become the melting point of the base film or higher. As a result, a part of the base film may melt. Even if it does not melt, it may be softened to increase frictional resistance between the film and a thermal head, whereby delivery of ribbon may be obstructed and give rise to such phenomena as temporary stopping of running at the thermal head or entanglement of the ribbon. This is the so-called sticking phenomenon, which is a serious problem in speedup of recording, and a base film with excellent heat resistance is needed for solving this problem.
The present invention solves such problems and provides a heat-sensitive transfer ribbon which is capable of performing heat transfer at high speed.
In a heat-sensitive transfer ribbon, in order to solve the problems of the prior art as described above, it has been considered to provide a heat-resistant protective layer on the back surface of the base film on which heat-fusible ink layer is provided. For example, as such a heat-resistant protective layer, it has been proposed to use an epoxy resin, a phenol resin or melamine resin, or to use a silicone resin, a fluorine resin, nitrocellulose or a polyimide resin, etc. (see Japanese Patent Publication No. 13359/1983).
The base film must be thin and strong, and a representative base film satisfying this condition is a polyester film.
For providing a heat-resistant layer comprising an epoxy resin or a phenol resin on such base film, it is a general practice to effect curing by heating after coating of the resin, but heat deformation occurs on the base film during the curing treatment according to such a method, and also smoothness of the surface is lost, whereby the film cannot have a uniform shape and properties as the heat-sensitive transfer ribbon.
On the other hand, in the case of using a silicone resin or urethane resin as the two-liquid type curable resin for avoiding high temperature heating, the time required for treatment becomes longer, or much labor is required for management of accurate control of the curing level (Japanese Laid-Open Patent Publication No. 196291/1984). Further, there is also a proposal to use a water-soluble resin as the heat-resistant protective layer, but in his case, since a water-soluble resin is employed, it is necessary to treat the surface of the coated film and crosslink the film to some extent by use of a crosslinking agent (Japanese Laid-Open Patent Publication No. 194893/1984).
We have made various studies in order to solve such problems, and our attention was drawn to the fact that a chlorinated product of a 4-methyl-1-pentene polymer which can be formed rapidly into a film at normal temperature to exhibit excellent heat resistance has been used and contained as the heat-resistant component in printing ink, and first investigated the separate coating of this film as the heat-resistant layer on the film surface of the polyester base film. However, this resin, while it can be caused to adhere onto a porous surface of cellulose type such as paper it cannot adhere at all onto a polyester film, and the film formed was also rigid and extremely brittle.
On the basis of a novel idea to use a 4-methyl-1pentene polymer which has been used only as the component contained in the ink composition of the prior art, singly as the component of the heat-resistant protective layer, we made further studies, and consequently found that by the use of a specific modified product of a 4-methyl-1pentene polymer or by the use of the modified product and a specific polyester resin in combination, a heat-resistant protective layer which has extremely firmly adhered onto the polyester base film surface and exhibits excellent heat resistance as well as excellent flexibility can be obtained.